<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
</body>
<script>
    var score = parseFloat(prompt('请输入学生成绩：'))
    var grade = score
    if(grade>=80){
        alert('该学生成绩等级为：A')
    }else(
        alert('该学生成绩等级为：B')
    )
</script>
</html>